module top_module (
    input [3:0] in,
    output reg [1:0] pos  );
    always@(*)
        begin
            case(1)
                 in[0]:pos=0;
                 in[1]:pos=1;
                 in[2]:pos=2;
                 in[3]:pos=3;
                 default:pos=0;
            endcase
        end
endmodule